1. Field of the Invention
The present invention relates to liquid crystal display devices. In particular, the present invention relates to an MVA (Multi-domain Vertical Alignment) liquid crystal display device.
2. Description of the Background Art
A typical liquid crystal display device realizes liquid crystal display in such a manner that a liquid crystal is held between two substrates at least one of which has an electrode and the electrode applies a voltage to the liquid crystal. Such a liquid crystal display device has been utilized in various uses because of its characteristics including low power consumption, small thickness, light weight and the like. In particular, an active-type liquid crystal display device that includes a TFT (Thin Film Transistor) serving as a switching element for selectively applying a voltage to liquid crystals on different domains in a screen allows high definition and, therefore, has been utilized for a television receiver, a monitor, a small-sized portable appliance and the like.
Display quality of a liquid crystal display device largely depends on an alignment state of a liquid crystal. A TN (Twisted Nematic) liquid crystal display device in which liquid crystals are aligned in a substantially parallel direction with respect to a substrate plane and a difference between alignment directions of the liquid crystals in substrate planes holding the liquid crystals (an angle of twist) is 90° has widely been used because of a higher CR (Contrast Ratio) obtained when a display screen is seen from a front side thereof and excellent production stability. However, the TN liquid crystal display device has the following problems. That is, the CR is remarkably degraded when a line of sight is displaced from the front side of the display screen, that is, a viewing angle becomes narrow. Further, color abnormality is caused due to tone reversal (a phenomenon that an actual display brightness does not increase/decrease monotonously with respect to a tone signal) occurring when the display screen is seen from a specific direction.
In order to solve these problems, a liquid crystal display device that utilizes alignment of a liquid crystal, which is different from that in the TN liquid crystal display device, has been developed and has been in practical use. As one example of such a liquid crystal display device, there is known a VA (Vertical Alignment) liquid crystal display device in which a liquid crystal is aligned in a substantially vertical direction with respect to a substrate plane upon application of no voltage to the liquid crystal.
In particular, there is known an MVA (Multi-domain Vertical Alignment) liquid crystal display device in which a structure that controls a tilt direction of a liquid crystal upon application of a voltage to the liquid crystal is formed in a form of a resin bump or by draft processing of an electrode to obtain divided plural domains which are different in liquid crystal tilt direction from one another. Herein, a display screen is divided into four domains which are different in liquid crystal tilt direction from one another by 90°, respectively, so that each domain is allowed to have a rotational symmetry of 90°. Thus, a viewing angle property can be made symmetrical in a vertical direction and a horizontal direction, so that a viewing angle can be widened (refer to, for example, Japanese Patent No. 2947350 (hereinafter, referred to as Patent Document 1)). Typically, display properties in a vertical direction and a horizontal direction are of importance to a display device. A liquid crystal display device is larger in viewing angle dependency than an electroluminescence display device such as a CRT (Cathode Ray Tube) or a PDP (Plasma Display Panel); therefore, an improvement in viewing angle dependency has been required. It is considered that the high symmetry of the viewing angle property in the MVA liquid crystal display device having the divided four domains is considerably excellent.
The MVA liquid crystal display device disclosed in Patent Document 1 has the following problem. That is, since light to be made incident on the liquid crystal is linearly polarized light, a transmittance of the light in a case where the display screen is seen from the front side thereof is degraded unless the liquid crystal is tilted at an angle of 45° with respect to a transmission axis of a polarizing plate. However, the light to be made incident on the liquid crystal is changed to circularly polarized light by a quarter-wave plate. As a result, when voltages to be applied to the liquid crystals are equal to one another, a single transmittance can be obtained when the display screen is seen from the front side thereof, irrespective of the tilt direction of the liquid crystal (refer to, for example, Japanese Patent Application Laid-Open No. 2002-303869 (hereinafter, referred to as Patent Document 2)).
Moreover, axially symmetrical alignment in which a tilt direction of a liquid crystal successively varies with a specific point being used as a symmetrical center realizes liquid crystal alignment which is considerably high in symmetry without degradation of a transmittance, leading to realization of a viewing angle property which is high in symmetry (refer to, for example, Japanese Patent No. 3875125 (hereinafter, referred to as Patent Document 3)). Herein, the axially symmetrical alignment does not correspond to a definition of MVA, that is, “division of domains” which are different in liquid crystal tilt direction from one another. However, the axially symmetrical alignment can be interpreted as a limit obtained by extrapolation of a divided number at infinity and, therefore, is included in the definition of the MVA in this specification.
As described above, an MVA liquid crystal display device that changes light to be made incident on a liquid crystal from a substrate normal direction to circularly polarized light is described as a circularly polarizing MVA liquid crystal display device in this specification. On the other hand, an MVA liquid crystal display device that changes light to be made incident on a liquid crystal to linearly polarized light is described as a linearly polarizing MVA liquid crystal display device.
Further, addition of a half-wave plate to a position between a polarizing plate and a quarter-wave plate makes it possible to change light to be made incident on a liquid crystal to circularly polarized light in a wide wavelength band, so that it is possible to decrease a brightness in black display in a case where a display screen is seen from a front side thereof and to enhance a CR in the case where the display screen is seen from the front side thereof. In a configuration on a backlight side and a configuration on an observer side, retardations of quarter-wave plates are made equal to each other, retardations of half-wave plates are made equal to each other, transmission axes of polarizing plates are made orthogonal to each other, slow axes of the quarter-wave plates are made orthogonal to each other and slow axes of the half-wave plates are made orthogonal to each other. Herein, a condition for enhancing a CR includes a fact that black display is performed upon application of no voltage to a liquid crystal (refer to, for example, Japanese Patent No. 3767419 (hereinafter, referred to as Patent Document 4)).
Further, an advantage of changing light to be made incident on a liquid crystal to circularly polarized light is to realize a semitransparent or transparent reflective VA liquid crystal display device capable of performing display in a reflection mode in which display is performed through use of outside light and a normal transmission mode in which display is performed through use of backlight in order to improve outdoor visibility (refer to, for example, Japanese Patent No. 3410663 (hereinafter, referred to as Patent Document 5)).
The circularly polarizing MVA liquid crystal display device described above has a feature in that liquid crystal alignment (axially symmetrical alignment) which is high in symmetry can be utilized without degradation in transmittance. In white display, however, there arises the following problem. That is, a transmittance is increased by enhancement of a retardation of a liquid crystal layer in such a manner that a thickness of the liquid crystal layer is made large, a birefringence of a liquid crystal material is made large or a voltage to be applied to a liquid crystal is made high, resulting in occurrence of tone reversal in a horizontal direction and a vertical direction of a liquid crystal display device.
In a conventional liquid crystal display device, a high CR, a wide viewing angle and an excellent symmetry have been required; therefore, a liquid crystal having a high symmetry must be aligned. In order to resolve the tone reversal described above, moreover, a transmittance must be decreased. In recent years, however, the liquid crystal display device has been utilized in a use as a liquid crystal display device for a vehicle or a liquid crystal display device for a ticket vending machine which does not necessarily require an isotropic viewing angle property, in accordance with improvement in durability of components and enlargement of a use temperature range. For example, the liquid crystal display device for a vehicle requires high display quality in a horizontal direction. However, display quality in a vertical direction is not necessarily higher than that in the horizontal direction because a positional relation between a head of a user and a panel is substantially fixed in the vertical direction. On the other hand, the liquid crystal display device for a ticket vending machine or an ATM does not require the display quality in the horizontal direction, but requires the display quality in the vertical direction in order to respond to change of a viewing angle in the vertical direction because the viewing angle in the vertical direction varies in accordance with a height and a standing position of a user.